The present invention relates to a solder strip exclusively for semiconductor packaging, and more particularly to a solder strip having solder holes arranged thereon in an improved pattern. The solder holes on the solder strip of the present invention have reduced diameter and properly increased density and are uniformly arranged in a specified pattern throughout the solder strip that is made of transparent plastic material, so that the solder strip may be used on a BGA carrier having any kind of metallic island layout to always provide sufficient number of solder holes relative to each metallic island and accordingly supply sufficient amount of liquefied solder on each island for bonding purpose.
Semiconductor devices typically include a plurality of components formed by photolithographic processes in a multi-layered structure. The structure is supplied with signals typically via a sunburst pattern that makes electrical contact to lands on the semiconductor device. For completing the packaging of the device, the sunburst pattern is connected to external signal and power sources for the device to operate and the device is protected from the environment.
One technology for completing the semiconductor package is termed a Ball Grid Array (BGA). The BGA comprises a planar component "carrier" that may comprise a rigid plastic layer with metallic (gold) islands or dots on a top surface thereof. The dots are connected to lands on a bottom surface of the carrier by through connections that provide electrical continuity from the power and signal sources to the metallic dots. The BGA, thus, is employed as a support or carrier for the semiconductor device and as a medium for providing the requisite drive and signals to the attached semiconductor device.
The semiconductor device, accordingly, has to be connected to the carrier in a manner to permit electrical continuity. To this end, it is necessary to provide solder on top of each of the gold dots on the top surface of the carrier to bond the semiconductor securely in place.
In recent years, there are various technical means developed to provide solder on the tops of dots on the carrier and to melt the solder to bond the semiconductor to the carrier. One of these technical means for bonding the semiconductor to the carrier is to place a solder strip in contact with the carrier on which the island or dot array is formed. A laser is scanned over the surface of the solder strip and pulsed in registry with the positions of each of the gold islands or dots. The solder is liquefied where impacted by the laser beam and thus, solder is transferred to the dots. Wherein, the laser can be raster scanned over the carrier and pulsed at each position of the islands or dots, or a laser diode array to supply a pulse to the solder strip over each row of islands or dots simultaneously. Or, a fiber fan can be used to supply pulses, a row at a time, from a single laser diode source. Or, the solder can be provided on the carrier at only selected ones of the islands or dots.
The solder strip used in the above-mentioned prior technical means includes a strip, that is, a carrier of the solder, that must be made of an insulating material or a material having very small coefficient of heat transfer, because any heat transfer and radiation effect of the strip would prevent the laser pulses from melting solder set in the solder holes on the strip, and the solder would not be transferred onto the islands on the BGA carrier as expected. Generally, the solder strip may be made of transparent plastic material, such as the rigid plastic sheet for making projection slide. The solder strip must also have adequate thickness and is provided with a plurality of holes into which solder paste is filled.
However, the above-mentioned solder strip has following limitations in its design and use:
1. Solder holes provided on the solder strip must be arranged in a pattern exactly corresponding to the layout of the islands or dots on the BGA carrier. When the BGA carrier has a different layout of islands or dots, a separate solder strip must be prepared to have a solder hole pattern consistent with the island or dot layout on the BGA carrier. Therefore, it necessitates disadvantageously increased cost to prepare and make the solder strip. PA1 2. In the process of transferring the solder to the islands or dots on the BGA carrier, it is a must to register the solder holes with the islands or dots on the carrier for a laser beam to direct at the holes and melt the solder paste therein. That is, the solder strip and the BGA carrier must be moved synchronously and the laser must be activated simultaneously. Although this process is not a problem in terms of the currently available automated apparatus for this purpose, it nevertheless increases limitations in the operation of such apparatus. PA1 3. The solder paste transferred from the solder strip to the islands on the BGA carrier is substantially in the form of a ball due to surface tension. Each of the solder balls must have specified height and size and it is a minimum requirement that the solder ball could never flood beyond the discrete islands. Therefore, the size of the holes on the solder strip, the amount of solder paste filled in the respective holes, and the accuracy of the registry of the holes with the islands on the BGA carrier all have important and critical influence on the transfer of the molten solder paste to the islands. That is, the manufacture of the solder strip is very troublesome.
Therefore, it is desirable to develop an improved solder strip to eliminate the above-mentioned disadvantages.